Rolling stock bogies on passenger and freight trains experience a range of stresses which cause their components to fail over time. Failures can be age related, but can also occur for a number of different reasons including operational conditions, manufacturing faults, heavy vehicle load, abnormal rail-wheel interface, poor lubrication, improper mounting or handling and weather conditions.
As a result, bogie health cannot be predicted purely based on factors such as age, and it is therefore important that bogies are monitored at regular intervals to ensure that any components that have failed or are about to fail are detected and replaced or repaired as required. The consequences of failing to detect a failed or failing component can be catastrophic, so it is important that such components are identified as quickly and as reliable as possible. As a result, it is known to employ trackside monitoring of bogies during use rather than relying only on periodic inspections of the rolling stock.
Each component has a normal operational temperature, acoustic and vibration emission which varies within reasonable bounds in operation. For example, the usual operating temperature for a wheel bearing is around 20° C. above ambient, while 70° C. or more above ambient would indicate a failure.
Known trackside monitoring solutions include Hot Axle Box Detectors (HABD) and Hot Wheel Detectors (WWD), both of which are temperature measurement devices that can be installed on trackside. A train detection system triggers a number of infrared probes/sensors which can be installed on either or both sides of the track as a train approaches. As the bogie comes into view of the sensor, the temperature at various parts of the bogie can be detected by the infrared sensors. A number of bogie components including wheel bearings, axle, brakes, motor and gearbox emit abnormal heat patterns when under stress or about to fail. Accordingly, the temperature readings can indicate faulty or failing components in wheels and bearings as well as, to a limited extent, faults in brakes, axles and/or other components.
The ability to rely on abnormal heat signatures generated during operation provides a clear benefit over static inspection of bogies, but there remain a number of significant shortcomings or disadvantages with this type of trackside monitoring.
Firstly, weather conditions can greatly influence the operation of a trackside system. Infrared detector capability is dependent on the intensity or amount of the received radiation, which can be heavily influenced by external environmental conditions such as snow, wind or rain. Alternatively, the infrared detection device may become saturated when directly exposed to sunlight or an intense reflection off a shiny surface, causing the detector to report artificially high heat levels. Strong sunlight can also lead to a piezoelectric effect which cases pyroelectric devices to produce false heat signals and report high levels of voltage.
Secondly, the detector area of these trackside devices is typically very small, often only 1 pixel or 2×2 pixels in resolution. As a result, the trackside sensors only generate point based measurements, providing no indication of how the heat is distributed across the component. Perhaps more significantly, there is a risk that the component of interest will not be scanned by the trackside detector. Unless the relevant component surface (wheel, bearing axle, brakes, etc) falls precisely within the detector's field of view, accurate readings cannot be made. Numerous different designs of bogie exist, with the size of wheels and the appearance and/or location of components varying between the designs. This leads to inconsistencies in the locations of interest and also potentially the lines of sight from a trackside location, which must be clear in order for accurate measurements to be made. Accurate timing is also necessary to measure a surface that is passing in front of the sensor at a very high speed relative to the static detector.
The static nature of trackside inspection systems also makes it all but impossible of them to acquire or maintain any data relating to how a particular bogie or component is performing. They can reactively sense a fault, but do not possess historic data for each bogie which could be used to any analytical tools for a better understanding the reasons behind the fault. Indeed, no information would typically be provided about where on a train a fault occurred, simply that a fault was present somewhere.
Finally, trackside devices can be relatively expensive to manufacture and install, so there will often be gaps of 4-5 miles, or in some cases up to 25 miles, between installations. Temperature increases in a failed bearing can be very rapid, eg from around 70° C. above ambient to around 300° C. above ambient in less than a mile. It is therefore quite conceivable that a failure could occur between installations, and not be detected in time for appropriate action to be taken.
These various factors can all lead to unacceptably high instances of missed detections and/or false positives.